Optical component with spectral separation

ABSTRACT

The invention relates to an optical component comprising at least one input guide element ( 1 - 4 ), at least one output guide element ( 5 ) and a spectral separation element ( 7 ) which is disposed between the input guide element(s) ( 1 - 4 ) and the output guide element(s) ( 5 ). The inventive component is characterised in that at least one of the input or output guide elements ( 1 - 5 ) comprises a fibre ( 1 - 5 ) containing a portion ( 21 - 25 ) which is designed to increase the mode range of a beam that it guides. According to the invention, the portion which is designed to increase the beam mode range can comprise a portion with a graded index, a portion having a core or gain size which varies radially and/or longitudinally, or a portion having a core or gain index which varies transversely and/or longitudinally.

The invention concerns the field of optical components and more particularly optical wavelength multiplexers and demultiplexers.

The person skilled in the art knows that it is possible to considerably increase the traffic of fiber optic networks through wavelength multiplexing and demultiplexing techniques. Each series of data to be transported is transmitted over a specific optical frequency, multiplying the capacity of the fiber by the number of wavelengths used.

FIG. 1 represents a multiplexer of the prior art. In this type of multiplexer, basic optical fibers 1 to 4 each dedicated to a frequency band have their end in a plane x constituting the input plane of the multiplexer. This multiplexer also comprises a collimation element 6 and a diffractive element 7. The input plane x of the multiplexer is confused with the focal plane of the collimation element 6 so that the input beams coming from the ends of the basic fibers 1 to 4 pass through the collimation element 6 and are located roughly parallel to one another. The diffraction element 7 is positioned so that the beams are sent back toward the collimation element 6, which superimposes them to introduce them at the end of a single outlet fiber 5.

As the rate of optical networks continues to increase and as the stability of the optical sources, particularly lasers, is not perfect, it is necessary to reduce the transmission fluctuations resulting from this instability by seeking a multiplexing bandwidth that is as wide as possible.

We therefore want to increase the ratio FWHM/Δλ where FWHM (Full Width at Half Maximum) designates the width of a basic band and Δλ designates the distance between two central wavelengths of two consecutive basic bands. We know that the ratio FWHM/Δλ is proportional to the ratio ω/Δx, where ω designates the mode range of the fibers and Δx designates the spacing between the fibers.

One known method for increasing the ratio FWHM/Δλ therefore consists in using a planar wave guide-based concentrator to reduce Δx. This integrated optical component in effect makes it possible to reduce the physical space between the paths. This type of concentrator is particularly well suited for use in planar wave guide AWG (Array Wave Guide Grating) multiplexers/demultiplexers. However, it is relatively expensive.

Another method consists in increasing ω. Thus, document EP 0 859 249 describes a fiber optics multiplexer comprising basic input fibers each carrying a frequency band and an array of microlenses, each microlens being associated with one fiber end. These microlenses make it possible to converge the beams coming from the basic fibers to produce parallel beams with a larger mode range that that of the input beams. The beams pass through a collimation lens that directs them toward a dispersion network making it possible to generate a single output beam consisting of different superimposed beams.

It will be understood that this technique requires precise positioning of the ends of the fibers with respect to the focal points of the microlenses as well as precise alignment of the axes of the basic fibers with respect to the focal axes of the microlenses.

One goal of this invention is to provide an optical fiber multiplexer/demultiplexer that is more economical, easy to assemble and has an enhanced FWHM/Δλ ratio.

To this end, the invention proposes an optical component comprising at least one input guide element, at least one output guide element and a spectral separation element positioned between the input guide element or elements and the output guide element or elements, characterized in that at least one of the input or output guide elements comprises a fiber comprising a portion designed to increase the mode range of a beam it guides.

Within the framework of this invention, the portion designed to increase the mode range of the beam may be formed of a portion with a graded index, a portion whose core or grain size varies transversally and/or longitudinally, or even a portion whose core or grain index varies transversally and/or longitudinally.

This type of optical component advantageously makes it possible to obtain a beam whose mode range is increased with respect to the mode range of the beam carried by the associated guide element.

In this component, the mode range expansion function of each beam is advantageously integrated in the input fiber and/or output fiber of this beam.

Other features and advantages will become clear upon reading the following description, which is purely illustrative and non-limiting in nature and which must be read in light of the appended figures in which:

FIG. 1 is a schematic representation of a multiplexer/demultiplexer with a diffractive element of the prior art,

FIG. 2 is a schematic representation of a multiplexer/demultiplexer that conforms to a mode of embodiment of the invention,

FIG. 3 represents a fiber example comprising a portion with a graded index,

FIG. 4 is a schematic representation of a multiplexer/demultiplexer that conforms to a second mode of embodiment of the invention,

FIG. 5 is a schematic representation of an embodiment variant of FIG. 4,

FIG. 6 is a schematic representation of a multiplexer/demultiplexer that conforms to a third mode of embodiment of the invention.

Of course this invention is not limited to the particular number of fibers illustrated in the appended figures, particularly to a 4 to 1 multiplexer, but extends to any component comprising n fibers.

The component represented in FIG. 2 operates like a multiplexer (this type of component could, of course, also be used in a demultiplexer). This component comprises coplanar optical fibers 1 to 5 that are parallel to one another and juxtaposed. Fibers 1 to 4 are input fibers, each dedicated to a given frequency band. Fiber 5 is an output fiber ensuring the transmission of the multiplexed optical beam obtained by superimposing the beams coming from the input fibers 1 to 4. The component also comprises a focusing element 6 of the lens type placed opposite the ends of fibers 1 to 5 and a diffractive element 7, for example a diffraction grating, that receives the signals coming from the input fibers 1 to 4 via the focusing element 6.

In known manner, the diffraction element 7 has the property of sending back the different wavelengths contained in the same incident beam separated angularly. Pursuant to the principle of light return, the grating can recombine the incident beams separated angularly and coming from the input fibers 1 to 4 in the direction of the output fiber 5 via the focusing element 6.

The input fibers 1 to 4 and the output fiber 5 have respectively at their end a silica portion 11 to 14 and 15 as well as an optical fiber portion with graded index 21 to 24 and 25. The portions of graded-index fiber have a core whose refractive index varies based on the radial distance. The refractive index, higher at the center of the core, decreases as we approach the optical cladding, thus forcing the light rays to follow a curved trajectory that periodically refocuses on the central axis of the core. In a graded-index optical fiber, the refractive index of the fiber changes according to a determined continuous index variation law, for example parabolic. Thus, the inclined beam moving away from the axis encounters an index environment that decreases progressively, which lays it down and brings it back toward the axis.

In this figure, the beams coming from the graded index portions travel in the void up to the focusing element 6, which focuses them on the element 7. Each beam corresponds to a given frequency range, and the grating is adapted to superimpose all the incident beams into a single beam directed toward the focusing element 6 and the output fiber 5.

In FIG. 3, we have illustrated more specifically an example of fiber 1 comprising a graded index portion. This type of fiber is formed of a classic monomode fiber 31 at the end of which has been soldered a silica segment 11 of length Ls followed by a graded-index fiber segment 21 with length Lg constituting the graded index portion. The beams coming from the core of the monomode fiber 31 successively pass through the pure silica segment 11 and the graded-index segment 21. In the pure silica segment 11, the beams have a tendency to diverge while in the graded-index silica segment 21, they have a tendency to reconcentrate. The work distance z_(ω) and the mode range ω of the beam leaving the fiber depend on the lengths Ls and Lg of the segments 11 and 21 soldered to the monomode fiber 31.

It is also possible to use similar fibers that do not comprise the pure silica segment 11. In this case, the monomode fiber 31 is directly soldered to the graded-index fiber segment 21.

Additionally, the fiber portions designed to increase the mode range of the beams are not necessarily constituted of added segments attached, for example, by soldering. We can also produce directly in the monomode fiber a fiber portion that generates an increase in the mode through local modification of the structure and/or of the properties of the fiber via an adequate treatment. We can, for example, to this end, use core expansion techniques (CET) via thermal diffusion to produce an expansion of the core of the monomode fiber over a portion of determined length.

In one implementation of the multiplexer of FIG. 2, the input fibers 1 to 4 and the output fiber 5 consists of monomode fibers 31 to 35 with a constitution similar to the one in FIG. 3.

In a variant of the multiplexer of FIG. 2 (not shown), the input fibers 1 to 4 consist of standard monomode fibers 31 to 34 similar to the one in FIG. 3. On the other hand, the output fiber 5 consists of a multimode fiber. In this variant, the mode range of the beam guided by the multimode fiber 5 is wider than the range of the beam guided by monomode fibers 1 to 4. The increase in the mode generated by the graded-index portions of fibers 1 to 4 makes it possible to obtain output mode ranges for these fibers adapted to the range mode of the multimode output fiber 5.

We assume that an “adapted” mode range means a mode range closer to the mode range of the multimode fiber 5 than the mode range of one of the monomode fibers 1 to 4.

The basic fibers 1 to 5 of FIG. 2 can be positioned in a fiber holder comprising V-shaped grooves for positioning the fibers. The ends of fibers 1 to 5 are then polished to be aligned with one another. The polishing operation slightly modifies the length of the graded-index fiber portions 21 to 25. We can show that this modification in length has few consequences for the mode range ω of the beam leaving the fiber.

Nevertheless, to precisely control the behavior of the beam, it is possible to add an additional silica segment at the end of each fiber 1 to 5 without any effect on the trajectory of the beams. The fibers are then positioned in the fiber holder with this additional segment before being polished together. In this way, the length of the graded-index portions 21 to 25 is not altered by the polishing operations.

We can also obtain enhanced performance by reducing the diameter of the fibers at their end. To this end, we can advantageously produce a chemical attack of their external surfaces to remove a layer of the optical cladding. We thus decrease the value of the spacing Δx between the mode ranges.

FIG. 4 is a schematic representation of a second mode of embodiment of the invention in which the spectral separation element 7 consists of multidielectric filters. These types of filters consist of a stack of thin layers of dielectric materials that reflect certain wavelength ranges and transmit others.

The input fibers 1 to 4 and the output fiber 5 are similar to the fibers of the device of FIG. 2. The separation element 7 concentrates the beams coming from the input fibers to send them to the output fiber 5.

According to FIG. 4, each layer of the spectral separation element 7 reflects one wavelength and transmits the others.

FIG. 5 is a schematic representation of a variant according to which, on the contrary, each layer of the spectral separation element 7 transmits one wavelength and reflects the others.

FIG. 6 is a schematic representation of a third mode of embodiment of a multiplexer/demultiplexer that conforms to the invention in which the separation element is of the guide grating type. This type of grating comprises two star couplers 10 and 12 separated by a guide grating 14. The guides have different lengths so that there is a step difference between two consecutive guides, which allows dephasing and therefore the multiplexing or demultiplexing of the beams passing through each of the guides. In this device, the fibers 1 to 5 consist of fibers similar to the fiber represented in FIG. 3.

The fibers used in the three preceding modes of embodiment make it possible to increase the mode ranges ω of the basic beams in integrated fashion. The device according to this invention has the advantage that the light beams do not pass through the air between the ends of the monomode fibers and the portions ensuring the mode range expansion function.

Furthermore, the invention makes it possible to eliminate the alignment problems inherent in the prior art.

Previously we described components according to the invention in which the portion designed to increase the mode range of the beam is formed of a portion with a graded index. However, the invention is not limited to this particular mode of embodiment. As indicated previously, this invention also extends to the case where the portion ensuring the mode increase is formed of a portion of fiber whose core or gain size varies longitudinally and/or transversally or even of a portion whose core or gain index varies longitudinally and/or transversally.

Additionally, it will be understood that the invention is not limited to the modes of embodiment previously described in which the spectral separation elements consist of a diffraction grating, multidielectric filters or a guide grating. For example, the diffractive element may consist of a “scale” grating, a volume-phase holographic grating, a prism, or even the combination of several of these elements. 

1. Optical component comprising at least one input guide element (1-4), at least one output guide element (5) and a spectral separation element (7; 10, 12, 14) positioned between the input guide element or elements (1-4) and the output guide element or elements (5), characterized in that at least one of the input or output guide elements (1-5) comprises a portion (21-25) designed to increase the mode range of a beam it guides.
 2. Component as claimed in claim 1, wherein the portion (21-25) designed to increase the mode range of the beam is formed of a graded-index portion.
 3. Component as claimed in claim 1, wherein the portion (21-25) designed to increase the mode range of the beam is formed of a portion of fiber whose core or gain size varies transversally or longitudinally.
 4. Component as claimed in claim 1, wherein the portion (21-25) designed to increase the mode range of the beam is formed of a portion of fiber whose core or gain index varies transversally or longitudinally.
 5. Component as claimed in one of claims 1 to 4, wherein the portion (21-25) designed to increase the mode range of the beam is formed of a segment of fiber added and attached to the end of the fiber (1-5).
 6. Component as claimed in one of claims 1 to 4, wherein the portion (21-25) designed to increase the mode range of the beam is formed by local modification of the structure and/or of the properties of the fiber, through an appropriate treatment.
 7. Component as claimed in any one of claims 1 to 6, wherein it forms a wavelength multiplexer/demultiplexer.
 8. Component as claimed in claim 7, wherein the spectral separation element (7; 10, 12, 14) receives the light beams from a plurality of input guide elements (1-4) and sends the superimposed beams to at least one output guide element (5).
 9. Component as claimed in claim 7, wherein the spectral separation element receives the superimposed light beams from at least one input guide element (5) and sends the beams separately to output guide elements (1-4).
 10. Component as claimed in one of the preceding claims, wherein the fiber or fibers (1-5) also comprise a pure silica portion positioned between the end of the fiber (31) and the fiber portion ensuring the increase in the mode range (21-25).
 11. Component as claimed in one of the preceding claims, wherein each basic fiber (1-5) comprising a portion designed to increase the mode range of the beam has at its end a protective silica portion connected to the fiber portion ensuring the increase of the mode range (21-25), said protective silica portions being polished to adjust their length.
 12. Component as claimed in one of the preceding claims, wherein the diameter of the end of the fiber or fibers (1-5) comprising at least one fiber portion ensuring the increase of the mode range (21-25) is reduced by chemical attack of the external surfaces of the fibers.
 13. Component as claimed in one of the preceding claims, wherein the spectral separation element (7; 10, 12, 14) is chosen from the group comprising diffraction gratings, multidielectric filters, guide gratings, scale gratings, volume-phase holographic gratings, prisms or even the combination of several of these elements.
 14. Component as claimed in one of the preceding claims, wherein a fiber of at least one input guide element (1-4) is a monomode fiber and a fiber of at least one output guide element (5) is a multimode fiber.
 15. Component as claimed in one of claims 1 to 13, wherein a fiber of at least one input guide element (1-4) is a multimode fiber and one fiber of at least one output guide element (5) is a monomode fiber.
 16. Component as claimed in one of claims 14 or 15, wherein the monomode fiber comprises a portion designed to increase the mode range of the beam, the beam thus having a mode range suited to the mode range of the multimode fiber. 